Question

there are 50 students in a class put of which 20 are girls . If a class representative (CR) is appointed , find the probability that the CR is a girl. what is the probability of the CR to be a boy ??

Answer 1

Total No. Of Obs = 50
No. of Girls = 20
No. of Boys = 50 - 20 = 30

1. Favourable obs of getting CR as girl = 20
    Probability of getting a CR as girl = Favourable Obs/Total No.Of Obs
    P(of getting CR a Girl) = 20/50
    P(of getting CR a Girl) = 2/5

2. Favourable obs of getting CR as boy = 30
Probability of getting a CR as boy = Favourable                                                                                              Obs/Total No.Of Obs
    P(of Getting CR a Boy) = 30/50
    P(of Getting CR a Boy) = 3/5
 
    Alternative Method
   
Probability of getting a CR as boy = 1 - Probability of getting a CR as girl
P(of Getting CR a Boy) = 1 - 2/5
P(of Getting CR a Boy) =  (5 - 2)/5
P(of Getting CR a Boy) =  3/5

U can use this method as Vice-Versa too

Answer 2

Total Number of students in the class: 50
Total Number of girls that can be chosen from: 20

Probability of a girl being chosen: 20/50= 2/5

Probability of a boy being chosen: 30/50= 3/5